In severe or chronic cases the disease of Diabetes is usually treated with injection preparations containing insulin, e.g. porcine insulin, bovine insulin or human insulin.
A number of different processes for the biosynthetic production of human insulin are known. Common to all of them is that the DNA strand coding for either the entire proinsulin, a modified form hereof or for the A and B chain separately is inserted into a replicable plasmid containing a suitable promoter. By transforming this system into a given host organism a product can be produced which can be converted into authentic human insulin in a manner known per se, cf. e.g. EP B1 85,083 or EP B1 88,117.
Some known processes for biosynthesis of proinsulin or similar insulin precursors and there conversion into insulin are described below.
Proinsulin may be prepared biosynthetically by using the method disclosed in the specification of European patent application No. 121,884. In this method the gene coding for proinsulin is inserted into a yeast strain and after culturing such transformed yeast train proinsulin can be isolated from the culture medium. Hereafter, proinsulin can be converted into insulin in a manner known per se. Yields of proinsulin obtained by this method are, however, unsatisfactory low for commercial production.
Insulin precursors of the formula B-X-A wherein B and A represent the B and A chain, respectively, of human insulin and X represents a polypeptide comprising at least 2 amino acid residues, preferably from 6 to 35 amino acid residues, are known from the specification of Danish patent application No. 5284/87. The precursors can be enzymatically digested into human insulin by treatment with trypsin and carboxypeptidase B in the presence of certain metal ions.
European patent application No. 195,691 discloses closely related insulin precursors of the formula B-X-Y-A wherein B and A represent the B and A chain, respectively, of human insulin, cross-linked through sulphur bridges as in human insulin, and X and Y each represents a peyine or arginine residue, as well as the preparation of said precursors. These precursors can be enzymatically digested into human insulin by treatment with trypsin and carboxypeptidase B. Moreover, said precursors can undergo tryptic digestion into des-B30-insulin; however, a considerable amount of A.sub.o Arg-des(B30)-insulin is formed which only slowly undergoes further digestion.